An example of a feedback control apparatus for controlling an ratio in an internal combustion engine is disclosed in Japanese Unexamined Patent Publication No. 60-240840.
This apparatus employs an oxygen sensor that detects the concentration of oxygen in exhaust. According to the oxygen concentration, the apparatus determines whether an actual air-fuel ratio is rich or lean relative to a theoretical air-fuel ratio. According to a result of the determination, the supply of fuel is feedback to controlled so that the actual air-fuel ratio becomes close to the theoretical air-fuel ratio.
When the actual air-fuel ratio greatly deviates from the theoretical one due to acceleration or declaration in the engine, the conventional feedback control system produces a large control factor to speedily attain the theoretical air-fuel ratio.
The feedback control system however, involves a response delay time. Namely, it takes time until a result of correction on the fuel supply according to the air-fuel ratio feedback control is detected by the oxygen sensor as a change in the oxygen concentration. This response delay time involves an intake air travel time, a gas residence time in a cylinder, delay of exhaust travel time, and a response delay time of the oxygen sensor itself. Accordingly, in the conventional feedback control system, the control factor is determined so that control time constants become approximately equal to these delay times, to converge the feedback control.
The response delay time in the conventional feedback control system hinders the control responsibility. If the deviation of an actual air-fuel ratio from a theoretical air-fuel ratio is large due to a transient operation, it takes a long time to attain the theoretical air-fuel ratio due to the response delay time in the feedback control system, and the quality of exhaust will deteriorate during this response delay time.